Neural mechanisms for stimulus selection in cortical areas of the macaque subserving object vision

The present article reviews some recent work on the neuronal mechanisms underlying space-based and feature-based stimulus selection in the primate occipito-temporal pathway of cortical visual processing. Clear evidence demonstrates that activity in areas V4 and IT is high for a stimulus which is selected either for its position in space or for its features, while it is considerably suppressed for other, irrelevant stimuli. Data are discussed within a conceptual framework whereby objects in the visual field always compete for focal resources. According to task demands, any kind of input (objects of a certain category, objects with a certain form, color or motion, objects at a certain location) can be behaviorally relevant. A short-term description (working memory) of the currently relevant object properties controls competitive bias in the visual system, such that inputs matching that description are favored to the disadvantage of task-irrelevant inputs. This framework emphasizes a tight, causal link between memory signals and mechanisms for stimulus selection in visual cortex. In all cases gating of neural activity was constrained by spatial factors. In area V4, responses to an ignored stimulus in the receptive field of the recorded neuron were maximally suppressed when the monkey attended to a second stimulus located within the boundary of the same receptive field, while suppression was virtually absent when attention was directed to a second stimulus well outside the receptive field border. In IT cortex, suppressed responses depended on both the selected and ignored stimuli being within the hemifield contralateral to the recorded hemisphere, while suppression was much reduced when the stimuli were presented across the vertical midline. These spatial constraints on the occurrence of modulation of visual responses may reflect limitations imposed by the local pattern of reciprocal inhibitory connections, which are supposed to underlie competitive interactions among objects in the field, that is among object representations in cortex.